Displays may be broadly classified into one of two types: emissive (e.g., CRTs and plasma display panels (PDPs)) or non-emissive. This latter family, to which liquid crystal displays (LCDs) belong, relies upon an external light source, with the display only serving as a light modulator. In the case of liquid crystal displays, this external light source may be either ambient light (used in reflective displays) or a dedicated light source (such as found in direct view displays).
Liquid crystal displays rely upon three inherent features of liquid crystal (LC) materials to modulate light. The first is the ability of LC materials to cause optical rotation of polarized light. Second is the dependence of such rotation on the mechanical orientation of the liquid crystal. Third is the ability of the liquid crystal to undergo mechanical orientation by the application of an external electric field.
In the construction of a simple, twisted nematic (TN) liquid crystal display, two substrates surround a layer of liquid crystal material. In a display type known as Normally White, the application of alignment layers on the inner surfaces of the substrates creates a 90° spiral of the liquid crystal director. This means that the polarization of linearly polarized light entering one face of the liquid crystal cell will be rotated 90° by the liquid crystal material. Polarization films, oriented 90° to each other, are placed on the outer surfaces of the substrates.
Light, upon entering the first polarization film becomes linearly polarized. Traversing the liquid crystal cell, the polarization of this light is rotated 90° and is allowed to exit through the second polarization film. Application of an electric field across the liquid crystal layer aligns the liquid crystal directors with the field, interrupting its ability to rotate light. Linearly polarized light passing through this cell does not have its polarization rotated and hence is blocked by the second polarization film. Thus, in the simplest sense, the liquid crystal material becomes a light valve, whose ability to allow or block light transmission is controlled by the application of an electric field.
The above description pertains to the operation of a single pixel in a liquid crystal display. High information type displays require the assembly of several million of these pixels, which are referred to in the art as sub pixels, into a matrix format. Addressing all of these sub pixels, i.e., applying an electric field to all of these sub pixels, while maximizing addressing speed and minimizing cross-talk presents several challenges. One of the preferred ways to address sub pixels is by controlling the electric field with a thin film transistor located at each sub pixel, which forms the basis of active matrix liquid crystal display devices (AMLCDs).
The manufacturing of these displays is extremely complex, and the properties of the substrate glass can be extremely important when producing displays having optimal performance. We have described some suitable substrate glasses in U.S. Pat. No. 6,060,168 to Kohli, U.S. Pat. No. 6,319,867 to Chacon et al., U.S. Pat. No. 6,831,029 to Chacon et al., and U.S. Pat. No. RE38,959 to Kohli. However, a need for glasses that can be used as substrates in the manufacture of active matrix liquid crystal display devices (AMLCDs) and other flat panel displays continues to exist, and the present invention is directed, in part, to addressing this need.
One technical issue facing the glass substrates for LCD displays, especially those displays made by high-temperature processes such as polysilicon technology, is the density change (compaction, or thermal stability) of the glass sheets after they are subjected to high-temperature treatment steps. The compaction of the glass sheets can lead to lack of registration of the semiconductor features created on the surface of the substrates, hence lower-quality or defective displays. Thermal stability of the glass sheet is dependent on the glass composition and thermal history thereof. Whereas a rigorously annealed glass sheet would have less compaction in down-stream processing, obtaining such thermodynamically stable glass sheet is difficult and could incur prohibitive costs to the manufacture process by requiring either a secondary heat treatment and/or a low production rate. It has been found that anneal point of the glass material correlates with the thermal stability of a glass sheet. For glass sheets produced by a given thermal process, the higher the anneal point of the glass material, the less the compaction of the glass sheets made therefrom.
The present invention addresses the various technical issues discussed supra.